This invention is concerned with the detection and location of targets hidden behind a surface, such as a surface of the earth, using continuous-wave radiation.
Although the detailed description of the invention refers to measurement of reflected electromagnetic radiation at microwave wavelengths, the principles of the invention are applicable to other types of continuous-wave systems, such as those using sound waves (e.g., sonar).
Various means and methods have been developed for detection and location of buried metallic and non-metallic objects which rely on the reflectivity of the objects at radar (microwave) wavelengths. These means and methods include devices which attempt to image the objects through beam scanning and determine distance (range) by timing differences between transmitted and reflected pulses (i.e., ground-penetrating radars) as well as devices which attempt to utilize expected differences between background (earth) reflectivity and the reflectivity of the buried object. Both methods are subject to significant difficulties in their ability to locate buried objects (especially non-metallic objects) due to several factors. These include:
(a) presence of other buried materials in surrounding soil (rocks, tree roots, etc.) whose reflectivities are comparable to that of the target object;
(b) rough or uneven terrain surface which produces widely-varying background reflected signals;
(c) for continuous-wave devices, constructive and destructive interference between transmitted and reflected waves;
(d) interference between multi-path reflected signals; and
(e) interference between the fundamental frequency and harmonics in the reflected wave.
Although pulsed devices which rely on timing are less subject to interference problems than continuous-wave devices, continuous-wave devices are inherently less complex, require less power, and may be made more easily portable.